The present invention relates to a process and a plant for the treatment of food products with ozone, the aim being especially, depending on the case, to bleach, disinfect or deodorize the products thus treated.
It is known that the literature on the ozone treatment of food products, in particular in the field of washing food products (seafood such as fish, crustaceans, but also fruit and vegetables . . . ) with ozonized water is extremely abundant--reference will be made especially to documents FR-385,815, EP-294,502, FR-797,928, CA-2,102,362 or U.S. Pat. No. 4,559,902.
One of the applications which is very promising today is therefore the washing of food products with ozonized water, in particular the washing of fruits and vegetables or seafood.
At the present time, chlorine in its various forms is the main disinfection auxiliary authorized in Europe. The two main forms of chlorine that can be used in such an application are gaseous chlorine, which requires very specific equipment but has, however, the advantage of being 100% active, and, on the other hand, hypochlorites (bleach) which are admittedly simpler to use but which, however, requires [sic] very delicate control of the doses.
The literature usually mentions that this control must be carried out particularly as a function of the quality and the pH of the water (too high a pH reducing the effectiveness of the hypochlorite), making it necessary to control the pH of the water typically between 6.5 and 7. In practice, industrial companies usually control the pH and the chlorine content of their water only locally, which results in the observation that, on the same washing line, chlorine contents can vary over a wide range about the desired content and can thus result in instances of nonconformity in terms of microbiological quality of the final products.
The drawbacks of using chlorine in the washing of food products are well known--they are especially related to the following factors:
the possibilities of water recycling are limited, since extensive recycling results in the formation of chloramines (which chloromines are suspected of having carcinogenic effects and of causing irritations in the eyes and airways) making it preferable to remove the residual chlorine by passing over an active carbon; PA1 chlorine is, moreover, a very stable element which persists for a long time in discharge water; PA1 finally, for user sites employing elemental chlorine, this is transported right to the industrial site in liquid form, which poses considerable safety and storage problems. PA1 washing with ozonized water does not require subsequent rinsing of the products (there are no remanant effects of ozone); PA1 any residual ozone which might persist in the water after washing will in fact react very quickly with the organic matter in the collected water--the industrial site will therefore discharge no residues, unlike in the case of chlorine; PA1 the ozone is produced on site, causing no drawback from the standpoint of transportation or storage; PA1 it has been demonstrated that ozonized water is effective over a wider pH range(typically between 6 and 8); PA1 for a plant provided with a cover and with a suitable extraction system for the washing tanks, the content of gaseous ozone in user manufacturing shops is less than the average exposure value (0.1 ppm), thus eliminating the irritation problems due usually to chlorine and chloramines; PA1 the use of ozone allows at least some of the washing water to be recycled. PA1 a) at least two initial solution containers, each container possibly being associated with a set value of the dissolved-ozone content that it is desirable to maintain in the initial solution in question; PA1 b) a source of a treatment gas mixture which contains ozone; PA1 c) a primary source of water; PA1 d) means for ozonizing a stream of water coming from said primary source of water with the aid of the treatment gas mixture, which means are capable of producing, as their output, a stream of ozonized water; PA1 e) means for injecting the stream of ozonized water into each of said initial solution containers; PA1 and is characterized by the combined implementation of the following measures: PA1 it includes a buffer tank which is fed with ozonized water coming from said ozonizing means and from which at least two transport lines capable of feeding each of said at least two initial solution containers with ozonized water leave, each transport line being provided downstream of the buffer tank with one of said flow-control devices; PA1 it includes, at least at one of the initial solution containers, a secondary line for feeding the container with make-up water, it being possible for each secondary line to feed the container in question independently or else for it to be connected in its downstream part to the feed/transport line from the initial ozonized-water solution container in question, downstream of the associated control device; PA1 it includes, at least at one of said initial solution containers, means for recirculating water from the container in question toward one or other of the following destinations: PA1 said ozonizing means include an ozonizing buffer tank which is fed with water coming from said primary source of water and from which at least two transport lines capable of feeding each of said at least two initial solution containers with ozonized water leave, each transport line being provided downstream of the ozonizing buffer tank with one of said flow-control devices, the ozonizing buffer tank being able to be fed with ozone according to one or other of the following configurations: PA1 at least at one of said initial solution containers, the plant comprises means for injecting a make-up gas containing ozone into the initial solution in question. PA1 a) at least two initial solution containers are used, each initial solution container being associated with a set value of the dissolved-oxygen content that it is desirable to maintain in the initial solution in question; PA1 b) a source of a treatment gas mixture which contains ozone is used; PA1 c) a primary source of water is used; PA1 d) means for ozonizing a stream of water coming from said primary source of water are used, these means being capable of producing, as their output, a stream of ozonized water; PA1 (e) a quantity of ozonized water is injected into each of said initial solution containers; PA1 characterized by the combined implementation of the following measures: PA1 at the exit of the ozonizing means, the stream of ozonized water is directed toward a buffer tank from which at least two transport lines intended for feeding each of said at least two initial solution containers with ozonized water leave, each transport line being provided, downstream of the buffer tank, with a flow-control device intended to carry out said control of the flow rate of ozonized water, each line feeding the initial solution containers; PA1 at least at one of said initial solution containers, a secondary stream of make-up water is added directly into the water in the container in question, or into the stream of ozonized water feeding the initial solution container in question coming from the ozonizing means; PA1 at least at one of said initial solution containers, water from the container in question is recirculated toward one or other of the following destinations: PA1 in case iii), the recycled water coming from said at least one of said initial solution containers undergoes, before its injection into the stream of water reaching the ozonizing means, a filtration step and, where appropriate, an ozonizing step; PA1 said stream of water coming from the primary source of water is ozonized, so as to obtain said stream of ozonized water, in the following manner: PA1 said stream of water coming from the primary source of water is ozonized, so as to obtain said stream of ozonizing water, in the following manner: PA1 at least at one of said initial solution containers, a stream of make-up gas containing ozone is injected into the water in the container in question.
It will therefore be understood in this context why both industrial companies and administrations in every country are seeking alternative solutions to chlorine for washing food products and that, among the alternatives available, ozone represents an attractive solution.
This is because:
The studies exhaustively carried out by the Applicant in this field have demonstrated the importance of maintaining a stable concentration of dissolved ozone in the water in the washing tank in order to obtain such disinfection (for example, a set point lying between 1 and 3 ppm depending on the products treated and on the passage time).
It is also necessary to take into consideration the concern that industrial companies usually have about recycling some of their washing water, especially in the case of washing some products such as salad vegetables.
Various solutions have been proposed in the literature for obtaining such a control of the ozone content in the water for washing food products, a first solution consisting in varying the power of the ozonizer.
This solution is certainly conceivable when a single washing line is being equipped, but if several lines are being equipped this solution requires an ozonizer and gas/liquid transfer system for each line.
Another solution envisioned by the literature consists in controlling a flow of ozonized gas (a single ozonizer and a system for controlling the flow of gas injected into the water feeding a given washing tank). Here again, the method proposed is not optimal, knowing that, on the one hand, it is then necessary to install a digital flowmeter for each line, that is to say for each washing tank, but that above all the gas/liquid transfer runs the risk of not being optimized insofar as these transfer systems (a static mixer or else a contacting tower) are designed for a given water flow rate and a given gas flow rate.
These conventional control systems are, for example, described in document CA-2,102,362 in the name of African Oxygen, which describes controlling the ozone content in the product treatment water by measuring the effective content in the water, comparing it with a set point and carrying out a feedback operation, where appropriate, depending on the result of the comparison, on the upstream means for dissolving the ozone.
It should also be noted that the effectiveness of these methods proves to be insufficient if there is a sudden ozone demand (for example, the case of finely shredded raw vegetables for which the exchange surface area is considerably increased) and that they do not take into account the question of water recycling either, since it would be necessary in this case to oversize each ozonizer not only to lower the ozone demand due to the organic matter in the water that is recycled but also to provide the necessary amount of ozone for disinfecting the food products.
The present invention aims to provide a technical solution to the abovementioned problems by making it possible in particular to operate with a single ozonizing unit, whatever the number of washing lines in service, and that the process can locally satisfy any sudden and strong demands for ozone, while still allowing the possibility of recycling at least some of the washings.
The studies exhaustively carried out by the Applicant have demonstrated that it was possible to come up with a technical solution to this combination of problems by producing a flow of overozonized water, that is to say in practice greater than or equal to the highest of the dissolved-ozone content set values associated with the washing tanks of the plant, and to control the dissolved-ozone content in each of the tanks with respect to each set value in question by controlling the flow of overozonized water transported by each feed line of the tanks.